1. Field of the Invention
The present invention relates to an auto-focus apparatus, a focus adjusting method, an image capturing apparatus and an image capturing method, and more particularly, is suitably applied, for example, to a video camera.
2. Description of the Related Art
Conventionally, video cameras contain a so-called auto-focus function which automatically performs a focusing operation of a lens in accordance with the distance to a subject (subject distance). For realizing such an auto-focus function, a variety of focus detecting methods have been devised for detecting a defocused state, and among others, an image processing method, an infrared method and a phase difference detecting method are representative.
The image processing method picks up a central region from an image captured by an imaging device (CCD: Charge Coupled Device), extracts high frequency components from the picked-up region, and adds the high frequency components to generate a value which is used as an evaluation value for detecting the focus. This evaluation value becomes higher as an image of a subject being captured approaches a focused state, and becomes lower as the image is further away from the focused state after it presents the highest value at the position of the focused state. Therefore, the image processing method moves the focus to examine whether the evaluation value increases or decreases, and adjusts the focus while moving the focus in a direction in which the evaluation value becomes higher until the focused state is reached. In other words, the image processing method performs a so-called hill climbing operation.
This image processing method is advantageous in that it can realize an auto-focus function without modifying or adding the design of the optical system such as lenses, and can improve the sensitivity to a focus error since the focus is adjusted using an image captured by the CCD.
Next, the infrared method applies the principles of triangulation to calculate the subject distance. Specifically, the infrared method irradiates an infrared ray from a video camera to a subject, detects an incident angle of return light reflected by the subject and returning to the video camera, and then calculates the subject distance based on the detected incident angle of the return light. The infrared method is advantageous in that the subject distance can be sufficiently measured, even if the subject is dark, as long as the amount of return light from the subject exceeds a predetermined amount, since the infrared ray emitted from the video camera itself is irradiated to the subject.
Further, the phase difference detecting method provides two sets of lens groups, each comprised of a small lens and a line sensor for detecting the position of light, in a lens optical system of a camera, and disposes the two sets of lens groups with their optical axes shifted from each other to realize the aforementioned triangulation. This phase difference detecting method is advantageous in that the capability of detecting a focusing state is constant irrespective of the subject distance.
The aforementioned image processing method, however, cannot detect a focusing stage unless the focus is moved to examine a change in evaluation value. Also, since the evaluation value varies in response to a small movement of a subject in the vertical direction with respect to the optical axis, the focused position can be erroneously detected. Therefore, the image processing method experiences difficulties in making the focus smoothly follow movements of the subject in the direction of the optical axis.
As a solution for the problems of the image processing method, the infrared method and the phase difference detecting method have been proposed. Since these methods can reveal a focusing state without moving the focus, they need not move the focus to examine. the focusing state. In addition, even if a subject moves in the vertical direction with respect to the optical axis, these methods will never erroneously measure the subject distance. However, because of its limited ability of measuring a distance of only about 10 m or less, the infrared method is not suitable for a business-use video camera which may capture a subject, for example, at a distance exceeding 10 m with a small depth of field (a range centered on the subject in which the subject is in focus).
Also, in the infrared method, since an optical system for emitting an infrared ray is generally disposed external to a video camera, the optical axis of the video camera cannot be aligned with the optical axis of the infrared ray, causing a problem of discrepancy between an actual screen range and a range viewed in a view finder, i.e., parallax.
FIG. 1 shows the principles as to how the parallax occurs. As shown in FIG. 1, a video camera 1 comprises a camera body 1A, and a camera lens 1B, an infrared ray emitter 1C and a return light incident angle detector 1D mounted on the camera body 1A, and applies the principles of triangulation to measure the subject distance.
Since the camera lens 1B is spaced from the infrared ray generator 1C by a predetermined distance, the optical axis A1 of an infrared ray is not coaxial with the optical axis A2 of the camera. In the video camera 1, since the optical axis A1 of the infrared ray is offset from the optical axis A2 of the camera in this way, even if a subject B1 to be captured is located on the optical axis A2 of the camera, the infrared ray may be irradiated to a subject B2 which is located on an axis offset from the optical axis A2 of the camera, i.e., on the optical axis A1 of the infrared ray.
In this event, the video camera 1 detects return light L1 from the subject B2, which is not to be captured, to measure the distance to the subject B2, and fails to measure the distance to the subject B1 to be captured.
The phase difference detecting method, on the other hand, suffers from a lower ability of measuring the subject distance as an iris of a camera lens is reduced. Specifically, since a video camera performs an auto-focus operation and an imaging operation simultaneously, the video camera cannot open the iris in the auto-focus operation and reduces the iris during the imaging operation, as does a still camera which separately performs the auto-focus operation and the imaging operation. Thus, due to the requirement of adjusting the iris during the imaging operation, the video camera cannot avoid a degradation in the ability of measuring the distance to a subject, resulting from the reduced iris.